Emissions Failures at MOT: What to Do Next

What the emissions test measures

At the MOT, the tester measures what is coming out of the exhaust. Petrol cars are checked for gases such as carbon monoxide and hydrocarbons, and diesels are checked for the smoke produced, which relates to soot. The tester also checks that emissions-related equipment such as the catalytic converter or diesel particulate filter has not been removed or tampered with.

Limits depend on the age and type of vehicle and are updated from time to time, so the failure sheet is the guide to what applies to your car.

A failure sheet will say whether the problem was the readings themselves, the visible condition of the exhaust or a missing component. Keep the sheet and take it with you when you visit a garage, because the numbers help narrow down where to look first.

Common causes on petrol cars

A petrol car that fails on emissions often has a fuelling or ignition fault. Worn spark plugs, failing ignition coils, a faulty oxygen sensor, an air leak or a clogged air filter can all upset the fuel mixture. A tired catalytic converter will also push readings up.

A car that has been used only for short trips may fail simply because the catalytic converter was not fully warmed up. Ask the tester how the car was prepared, and note whether the engine was warm.

Engine warning lights count as well. A light for the engine management or emissions system that stays on can be a problem in its own right at the test, regardless of what the tailpipe readings show.

• Worn spark plugs or failing coils
• Faulty oxygen sensor
• Exhaust leak before the sensor
• Ageing or contaminated catalytic converter
• Fuel mixture problems

Common causes on diesel cars

Diesels most often fail on smoke. Typical causes are a blocked or removed diesel particulate filter, a clogged or sticking EGR valve, dirty or worn injectors, turbo problems, or a blocked air intake.

A diesel that is regularly used for short journeys can build up soot and carbon deposits, and the DPF may never complete a regeneration. A longer steady drive before the test sometimes helps, though if a fault is present it will not cure it.

Dirty or blocked air filters and long-overdue servicing also raise smoke and emissions in older diesels, so it is worth checking the service history before assuming something major has failed.

Why diagnostics should come first

It is tempting to try a fuel additive or a new part and retest. Without diagnosis you may spend money on the wrong thing. Emissions diagnostics reads fault codes, looks at sensor data and measures the exhaust with an analyser to pin down the cause.

At Banks of the Tyne in Hebburn we carry out emissions diagnostics, and can go on to exhaust repairs, catalytic converter replacement or DPF cleaning and repairs as the findings require. We explain what we find before any work starts.

The analyser measurements are compared against what the engine management should be doing. If the sensors say the mixture is rich but the tailpipe says lean, for example, that mismatch points to a sensor or an air leak rather than a worn engine, and it is only by testing that the difference shows.

Repair routes and what happens with the retest

Once the cause is fixed, the car is retested. Rules about retests, timescales and whether a partial retest is available change from time to time, so check the current MOT guidance or ask the test station.

Repair routes vary: a service item such as plugs or an air filter, a sensor or valve, cleaning a filter, repairing the exhaust, or replacing a converter. Cost depends on the diagnosis and the make and model, so ask for an estimate once the cause is known.

Do not be tempted to remove emissions equipment such as a filter or converter to get the car through. That is likely to be an MOT failure, and rules on tampering with emissions systems are strict, so check them before considering anything of that kind.

Frequently asked questions